Lightguide fiber is drawn from a solid glass cylinder, or preform. The preform having a central core surrounded by a cladding material may be fabricated by the modified chemical vapor deposition (MCVD) process which is described in an article titled "Lightguide Preform Manufacture" in the Western Electric ENGINEER, by Partus and Saifi, Vol. XXIV, No. 1, pages 39 to 47, dated 1980 which is incorporated herein by reference. Layers of fused doped silica are built-up on the inside of a long silica glass tube by the reaction of glass precursor vapors, resulting in the formation of particles which deposit and are fused on the inner wall of the tube. The composition of the reactant vapors is automatically controlled to give a step or graded index of refraction in the deposited glass layers which will form the core of the preform. When a fiber is drawn from the preform, the deposited glass becomes the lightguide fiber core and the silica glass tube becomes the fiber cladding.
In particular, vapors of materials such as GeCl.sub.4, SiCl.sub.4, POCl.sub.3 or the like are entrained in a carrier gas such as oxygen and are drawn as a vapor stream into the interior of the glass tube which is rotated as a torch repeatedly traverses its length. As the vapor stream passes through the tube and encounters a heat zone adjacent the torch it reacts creating oxides which deposit on the interior surface of the tube. After numerous traversals of the torch along the length of the tube to deposit said layers, the tube is then subjected to elevated temperatures (e.g., 1900.degree. to 2000.degree. C.) by the torch in several traversals to shrink the tube and in a final traversal the tube is collapsed, resulting in a solid rod shaped preform.
A problem arises when the material of the core layers contains a volatile dopant such as germania and/or phosphorous pentoxide wherein the elevated temperatures necessary to soften the tube wall during the shrinking and collapse traversals can cause volatilization and loss of the dopant from the deposited core layers. This will undesirably change the dopant concentration therein which alters the refractive index profile of the core of the resulting preform.
U.S. Pat. No. 4,165,224 to Irven et al. attempts to solve the volatilization problem by passing a gas mixture containing oxygen and a halide or oxy-halide of the element having the volatile oxide through the tube during the shrinking traversals. At the hot zone the chloride and oxygen react to produce germania and chlorine. The germania so formed tends to dissociate and form germanium monoxide and oxygen. The excess germanium monoxide tends to drive the equilibrium volatilization reaction into reverse thereby suppressing the loss of germania from the surface region of the bore of the tube.
Although such a technique has been found to be most effective in improving the index profile of the preform core, during the collapse traversal of the tube, the flow of the halide and oxygen to the inside of the tube has been intentionally shut off, as it is no longer possible to flow gases therethrough. Accordingly, volatilization of the germanium monoxide and/or germania is allowed to take place during the collapse traversal which deleteriously alters the concentration of germania in the inner layer of the core of the lightguide preform resulting in a dip in the refractive index.